1. Field of the Invention
This invention relates to a multi-piece solid golf ball having satisfactory flight performance, spin properties, feel, durability, and mass-scale productivity.
2. Prior Art
Two-piece solid golf balls are widely used. For amateur golfers, two-piece solid golf balls have the advantage of distance, but the disadvantages of a long run, difficulty of control, and a hard feel. The feel of two-piece solid golf balls can be softened by reducing the compression thereof, although this attempt sacrifices resilience and hence, flight distance.
One solution to overcome these drawbacks is to modify the ball structure into multi-piece solid golf balls. More particularly, the solid core portion is divided into two or more layers, and an appropriate hardness, specific gravity and diameter or gage are assigned to the respective layers.
However, prior art multi-piece solid golf balls still leave some problems to be solved with respect to their manufacturing process and lack mass-scale productivity. For example, in the case of three-piece solid golf balls consisting of a two-layer solid core and a cover, both the inner and outer layers of the solid core are formed of rubber compositions comprising polybutadiene, a metal salt of an unsaturated carboxylic acid and a peroxide. In preparing the solid core from these rubber compositions, the core inner layer is first formed by molding the inner layer composition under heat and pressure in accordance with a technique for forming the core of two-piece solid golf balls. The core inner layer is then enclosed within the core outer layer by preforming the outer layer composition into half shells in an unvulcanized or semi-vulcanized state, encasing the core inner layer in the shells, and molding them under heat and pressure. This process requires twice or more steps than the number of steps used in the manufacture of the core of conventional two-piece solid golf balls.
It is possible to mold and cure the rubber composition for the core outer layer by an injection molding technique. However, injection molding is impractical partially because polybutadiene and analogous rubber are poor in flow as is well known in the art, and partially because curing reaction is effected during molding so that the cycle time is prolonged.
Further, JP-A 244174/1992 discloses a three-piece golf ball comprising an elastomeric core, an intermediate layer, and a cover of thermoplastic material wherein the intermediate layer is formed of a thermoplastic resin composition containing at least 10% by weight, preferably at least 35% by weight of an amide block copolymer. JP-B 8301/1995 discloses a three-piece golf ball having an intermediate layer made of thermoplastic polyester elastomer. These golf balls, however, are still insufficient in spin properties. Especially the spin performance under a situation where large shear stresses are applied to the ball as on full shots with an iron is inferior, as compared with the above-mentioned multi-piece solid golf balls in which the solid core of two or more layers made of polybutadiene rubber base compositions is enclosed within the cover.